High pressure discharge lamps which contain a fill which includes metals generate, in operation, positive metallic ions, see the referenced British patent 1,233,955. These positive metal ions may diffuse through the wall of the discharge vessel. If current carrying leads are located in the vicinity of the discharge vessel, the electrical field which is generated enhances ion migration in operation of the lamp. The electrical field also draws photoelectronsout of the vessel, which photoelectrons are generated by ultraviolet (UV) radiation during a discharge within the discharge vessel. The photoelectrons face toward the wall of the discharge vessel and enhance ion migration. The diffusion process leads to losses of the metallic fill additives, and, thus, the composition of the fill will change and result, eventually, in unstable behavior of the discharge arc within the high pressure discharge lamp.
Migration of ions occurs in many lamps, but particularly in high pressure discharge lamps which have discharge vessels made of quartz glass and which have metal halide additives in the fill. In operation, and due to the discharge, metal ions having a small ion radius, such as, for example, sodium ions will be generated. During operation of the lamp the external electrical fields enhance migration of the positively charged sodium ions through the wall of the vessel. The results will be that an excess of halide, particularly iodine, will remain within the discharge vessel. The excess iodine has electronegative behavior. This results in difficulties in igniting the discharge lamp, and raises the operating voltage of the arc tube.
The aforementioned referenced British patent 1,223,955 teaches the use of a shielding electrode in order to solve these problems. Merely placing a shielding electrode into the lamp, however, raises another problem namely that of electric arc-over between the shielding electrode and the current supply leads which extends parallel thereto. The outer bulb, in accordance with the British patent, can be filled with an inert gas which may have additives therein which are electronegative in order to decrease the average or median free path length of the photoelectrons.
It has been found, in operation, that electric arc-over between the shielding wire and the parallel current supply leads cannot be reliably prevented. Filling the outer bulb with a gas decreases the heat insulation of the discharge vessel. Further, the shielding wire, necessarily, is self supporting and free in space. Upon vibration, the shielding wire may deflect or oscillate, and hit against the current supply lead or against the discharge vessel, causing damage thereto, or even destruction thereof.
The referenced U.S. Pat. No. 4,843,266 describes a single-ended discharge lamp having an outer bulb and an axially located double-ended discharge vessel in which the current supply lead extending to the end remote from the base is insulated, for example, being retained within a glass sleeve. This lamp does not have a shielding electrode, and the insulating cover for the current supply lead does not have any shielding function with respect to the electrical field, or to inhibit migration of the ions which are generated due to the discharge. Ion migration is somewhat reduced, but not suppressed or inhibited to the extent desired.